This invention relates in general to electrostatography, and more specifically, to a process for preventing imaging member belt set in an imaging machine environment.
Flexible electrostatographic belt imaging members are well known in the art. Typical electrostatographic flexible belt imaging members include, for example, photoreceptors for electrophotographic imaging systems, electroreceptors such as ionographic imaging members for electrographic imaging systems, and intermediate image transfer belts for transferring toner images in electrophotographic and electrographic imaging systems. These belts are usually formed by cutting a rectangular sheet from a web containing at least one layer of thermoplastic polymeric material, overlapping opposite ends of the sheet, and welding the overlapped ends together to form a welded seam. The seam extends from one edge of the belt to the opposite edge. Generally, these belts comprise at least a supporting substrate layer and at least one imaging layer comprising thermoplastic polymeric matrix material. The "imaging layer" as employed herein is defined as the dielectric imaging layer of an electroreceptor belt, the transfer layer of an intermediate image transfer belt and, the charge transport layer of an electrophotographic belt. Thus, the thermoplastic polymeric matrix material in the imaging layer is located in the upper portion of a cross section of an electrostatographic imaging member belt, the substrate layer being in the lower portion of the cross section of the electrostatographic imaging member belt.
Flexible electrophotographic imaging member belts are usually multilayered photoreceptors that comprise a substrate, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer and, in some embodiments, an anti-curl backing layer is desirable for imaging member flatness. Optionally, an overcoating layer may also be formed over the charge transport layer to provide wear protection. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. A typical layered photoreceptor having separate charge generating (photogenerating) and charge transport layers is described in U.S. Pat. No. 4,265,990, the entire disclosure thereof being incorporated herein by reference. In one embodiment, the charge transport layer is located at the top and over the charge generating layer of the imaging member. In an alternative embodiment, the charge generating layer is positioned on top of the charge transport layer. The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer.
Although excellent toner images may be obtained utilizing multilayered seamed belt photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, fatigue induced cracking of the charge transport layer and cracking initiation at the welded seam area were frequently encountered during photoreceptor belt cycling. Seam cracking initiation has also been found to rapidly propagate into catastrophic seam delamination as a result of continuing imaging belt fatigue which shortens belt service life. Dynamic fatigue induced imaging layer cracking and seam cracking and delamination may also occur in ionographic imaging member belts and intermediate image transfer belts.
The seamed flexible electrostatographic imaging member belt is usually fabricated from a sheet cut from a web. The sheets are generally rectangular in shape. All edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheets are formed into a belt by joining overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing, and the like. Ultrasonic seam welding is generally the preferred method for joining imaging member belts because it is rapid, clean (no solvents) and produces a thin and narrow seam. Another reason that the ultrasonic seam welding process is preferred is because it causes generation of heat at the contiguous overlapping end marginal regions of the sheet to maximize melting of one or more layers in the contacting overlapped ends of the imaging member sheet, which facilitates direct substrate to substrate fusing at the overlapped ends to form a seam having strong bond strength. For reason of simplicity, the discussion hereinafter will focus primarily on electrophotographic imaging members as a representation of electrostatographic imaging members.
When the overlapped ends of the cut sheet are ultrasonically welded to form a belt, the seam of the flexible multilayered electrophotographic imaging member, due to material discontinuity and excess localized seam thickness, can initiate crack formation and eventually delaminate during extended bending and flexing over small diameter belt support rollers of an imaging machine or when subjected to lateral forces caused by rubbing contact with stationary web edge guides of a belt support module during cycling. Mechanical failure due to seam cracking and delamination is further aggravated when the belt is employed in an electrophotographic imaging system utilizing a cleaning device such as a cleaning blade. Alteration of materials in the various photoreceptor belt layers such as the conductive layer, hole blocking layer, adhesive layer, charge generating layer, and/or charge transport layer to suppress cracking and delamination problems is not easily accomplished because alteration may adversely affect the overall electrical, mechanical and other properties of the belt such as residual voltage, background, dark decay, flexibility, and the like.
For example, when a flexible imaging member in an electrophotographic machine is a photoreceptor belt fabricated by ultrasonic welding of overlapped opposite ends of a sheet, the ultrasonic energy transmitted to the overlapped ends melts the thermoplastic sheet components in the overlap region to form a seam. The ultrasonic welded seam of a multilayered photoreceptor belt is relatively brittle and low in strength and toughness. The joining techniques, particularly the welding process, can result in the formation of a splashing that projects out from each side of the seam in the overlap region of the belt. Because of the seam overlapping and the seam splashing, a typical flexible imaging member belt is about 1.6 times thicker in the seam region than that of the remainder of the belt for example, a typical belt thickness is about 116 micrometers, reference Example I, whereas the overlapping seam region can be about 186 micrometers.
The photoreceptor belt in an electrophotographic imaging apparatus undergoes bending strain as the belt is cycled over a plurality of support and drive rollers. The excessive thickness of the photoreceptor belt in the seam region due to the presence of the splashing results in a large induced bending strain as the seam travels over each roller. Generally, small diameter support rollers are highly desirable for simple, reliable copy paper stripping systems in electrophotographic imaging apparatus utilizing a photoreceptor belt system operating in a very confined space. Unfortunately, small diameter rollers, e.g., diameter less than about 0.75 inch (19 millimeters), raise the threshold of mechanical performance criteria to such a high level that photoreceptor belt seam failure can become unacceptable for seamed multilayered belt photoreceptors. For example, when bending over a 19 millimeter diameter roller, a typical photoreceptor belt seam splashing may develop a 0.96 percent tensile strain due to bending. This is 1.63 times greater than a 0.59 percent induced bending strain that develops within the rest of the photoreceptor belt. Since the 0.96 percent tensile strain in the seam splashing region of the belt represents a 63 percent increase in stress placed upon the seam splashing region of the belt, seam cracking and delamination will occur prior to the onset of other photoreceptor belt mechanical failures and become limiting factors that determines the functional life of the belt. Under dynamic fatiguing conditions, the seam provides a focal point for stress concentration and becomes the initiation site for premature material failure, which adversely affect the mechanical integrity of the belt. Thus, the seam overlapped thickness plus the splashing tend to shorten the mechanical life of the seam and service life of the flexible member belt in copiers, duplicators, and printers. In addition to the seam cracking and delamination problems, a negatively charged photoreceptor belt has also been observed to exhibit charge transport layer fatigue cracking failure as a result of repeating tension stress in the charge transport layer when the belt bends and flexes over each belt module support roller during dynamic belt cycling.
In addition to all the above-mentioned mechanical failures, seamed electrophotographic imaging member belts have been found to encounter still another major physical and mechanical shortfall under actual machine operating conditions. This shortfall is manifested as localized imaging member belt set corresponding to each location where the belt makes parking contact with belt module support rollers after each prolonged machine idle period. In a service environment, an imaging member belt mounted on a belt support module is frequently activated into cyclic motion whenever the electrophotographic imaging process is initiated during the working hours of a work week. In reality, these imaging machines are rarely continuously in copying use. It is common for them to sit idle for relatively long periods of time in a ready mode (awaiting next instruction to print an image) and a power saving mode where most of the machine stations are inactivated or otherwise turned down to reduce power consumption. For example, corona generators are generally disabled, and the fuser station in some machines is disabled (i.e., power is cut off and the fuser station cools), while in others the power is reduced to lower the fuser station temperature to a standby temperature to enable relatively rapid return to process temperature. However, during the majority of the time, the machine is idle. While the machine is idle, the belt is stationary and parked over the belt support module rollers. Prolonged stationary parking of the belt while the machine is idle causes development of belt set sites. The time period of idle belt parking can often be extensive, particularly at night, on holidays, and over each week end. Since the imaging member belt comprises layers of thermoplastic polymer materials, it has an inherent propensity to exhibit creep compliance in response to any externally imposed stresses induced by the bending tension and compression effects on the top region and bottom region, respectively, of a segment of the belt while the belt is directly parked over each belt module support roller. After extended periods of machine idle time, each belt segment parked over a support roller develops a set.
A photoreceptor belt set is defined as a characteristic exhibition of localized permanent material deformation caused by an irreversible process of molecular chain slippage in chain entanglement sites which thereby reduces the degree of entanglement in response to the direction of induced bending stress of the affected segment of the belt. The direction of induced bending stress is in conformance to the surface curvature of the belt support roller over which the segment of the belt is bent. The sites are manifestations of physical deviations from the required photoreceptor surface flatness. Belt set in the electrophotographic imaging zone is undesirable because each set creates a small surface protrusion mound or ridge with two adjacent valleys which adversely affect photoreceptor charging uniformity as well as the efficiency of transfer of toner image to paper due to an inability of the photoreceptor to make even and intimate surface contact with the paper. Therefore, the sites of belt set degrade final copy print quality. Since reduction of localized polymer chain entanglement density within a set is a result of irreversible inter polymer chain motion, the sites of belt set induced in the imaging zones affect the belt surface uniformity and also cause early onset of fatigue charge transport layer cracking, as a consequence of interchain separation or total disentanglement which appears as cracks in the coating layer under repeated tension stress. Moreover, if a set is present in the seam region, it hastens the development of seam cracking initiation and seam cracking propagation leading to catastrophic seam delamination during photoreceptor belt machine cycling. Furthermore, the sites of photoreceptor belt set adversely impact belt transporting motion quality, interfere with cleaning blade functions, reduce the critical gap dimension between the belt imaging surface and subsystems such as closely spaced developer applicators, and adversely affect the driving efficiency of a drive roller or rollers for the photoreceptor belt.
Electrophotographic imaging devices also comprise subsystems which generate contaminants which degrade the life of photoconductive imaging members having a surface adjacent to the subsystems. In U.S. Pat. No. 5,376,990, a printing machine is described which includes a second driving mechanism to further drive the imaging member during power down and standby modes, and other critical periods, so as to extend the life of the imaging member by providing a uniform aging effect to the belt. The driving of the photoreceptor is conducted for a preselected duration of time following power off or power saver modes to allow the contaminants, e.g., ozone, nitrous oxide, heat, etc., generated by the subsystems to dissipate whereby the photoreceptor is uniformly aged. Fifteen minutes is generally sufficient time for driving according to U.S. Pat. No. 5,376,990 and ten minutes is preferred. The entire disclosure of U.S. Pat. No. 5,376,990 is incorporated herein by reference. Driving of the photoreceptor belt for a predetermined period of time until contaminants, e.g., ozone, nitrous oxide, heat, etc., generated by the subsystems to dissipate, e.g. 15 minutes, following power off or power saver does not take into consideration the fact that the belt remains stationary on support members such as rollers for many hours after the contaminants generated by the subsystems have dissipated and after the predetermined period of time has expired. The objective of U.S. Pat. No. 5,376,990 is to drive the belt for a pre-selected period of time during machine idling to facilitate uniform imaging member belt aging caused by chemical effects. For example, if a machine is shut down overnight from 6:00 PM to 7:00 AM (13 hours) and the belt is driven for 15 minutes after shut down, the belt would have been stationary for 12 hours and 45 minutes, a time 5,100 percent greater than the brief predetermined driving period after shut down. If shut down extends over a weekend for a period from 6:00 PM Friday evening to 7:00 AM Monday morning, the belt would have been stationary for 54 hours and 45 minutes, a time 29,900 percent greater than the brief predetermined driving period after shut down. The belt photoreceptor will form a set during these long periods of machine shut down.
Under a typical ambient room temperature of about 25.degree. C. and about 37 percent relative humidity, a belt will form an undesirable permanent set when the given segmental area remains parked over a belt support roller for approximately 48 hours (2,880 minutes). This permanent set conforms, to a notable degree, to the shape of the roller on which the belt segment is parked. Since a small roller such as a 19 millimeter diameter roller induces large bending strain/stress in a belt, prolonged belt parking while a machine is idle will exacerbate belt set. Other crucial environmental conditions that have a strong impact on belt set during prolonged belt parking while an imaging machine is idle are factors such as elevated temperature and high humidity. It has been found that a set which exhibits a diameter of curvature of more than about 3 inches does not normally manifest itself into copy print out defects, change the belt surface, alter machine subsystems tolerance, or interfere with cleaning blade functions because the set will effectively be pulled straight under an applied tension of one pound per inch width of belt. However, an imaging member belt with a set having a diameter of curvature of less than about 3 inches can impose problems because the set will exhibit a conspicuous projecting it rounded ridge with adjacent valleys at either side of the mound in the belt surface, all traversing the full width of the belt, when supported in a flat configuration for electrical charging and toner image to paper transfer during electrophotographic imaging processes. Since imaging belt set is a characteristic of interpolymer chain slippage under imposed stress, it is an irreversible process of chain disentanglement which reduces chain entanglement density. Thus, the belt segment location of the set becomes the site for early development of fatigue induced surface cracking.
Therefore, there is an urgent need for improving the physical and mechanical characteristics of seamed flexible imaging member belts to prevent the development of belt set and its associated problems, withstand greater dynamic fatiguing conditions, extend belt service life, as well as overcome any of the previously described shortfalls.